In the process of producing a semiconductor device, ion implantation and pattern etching are performed using resist patterns as a mask.
However, it is known that a resist pattern obtained by lithography and a transfer pattern formed by an etching after that suffer from variations in dimensional precision due to process conditions, the density of arrangement of patterns, underlayer conditions, and other various factors. This variation in dimensional precision becomes a factor causing short-circuits between patterns, disconnection, and other defects.
Therefore, in recent years, for obtaining a transfer pattern closer to a design pattern, so-called “optical proximity effect correction” (hereinafter referring to as “OPC”) has been performed to correct the design pattern to obtain a mask pattern for exposure (that is, “exposure mask pattern”). From this, model-base OPC is a method, as shown in FIG. 11, of giving edges of a design pattern 1, shown by a broken line in the figure, edge division points P, performing simulation of light intensity using the centers between edge division points P as evaluation points, automatically determining amounts of correction for the edge positions between the edge division points P based on the obtained simulation results, and thereby forming the exposure mask pattern 3.
The shape of the exposure mask pattern 3 given by the above model-base OPC is determined by an interval t between the edge division points P. If the interval t is large, the correction becomes that much rougher, and therefore improvement of the precision of the transfer pattern obtained by lithography using the exposure mask pattern is difficult. As opposed to this, if the interval t between the edge division points P is small, the exposure mask pattern 3 ends up becoming complicated, production becomes difficult, and the production cost of the mask increases. Further, due to the occurrence of fine patterns, defect inspections become impossible and sometimes fabrication of a mask guaranteed against defects becomes impossible. Therefore, in model-base OPC, the interval t between the edge division points P is set so as to secure the shape precision of a transfer pattern and to enable fabrication of a mask guaranteed against defects.
Further, when giving edge division points P to a design pattern 1, as shown in FIG. 12, using the vertexes P0 of the design pattern 1 as start points, edge division points P are successively given in accordance with the set interval t.
However, when using the vertexes of a design pattern as start points and successively giving edge division points, the interval between the position of a start point and an edge division point may result in “deviation” in the positions of the edge division points at facing edge portions. For example, in the L-shaped design pattern 1 shown in FIG. 12, if using the vertexes of the design pattern 1 as the start points P0, setting the interval t, and successively giving edge division points P, as long as the interval t does not match with the line widths W1 and W2 of the design pattern 1, as illustrated, “deviation” will occur at the positions of the edge division points P at facing edge portions.
In this case, as shown in FIG. 11, the edge portions of the exposure mask pattern 3 obtained by correction of the design pattern 1 become shifted in a step manner between facing edge portions. Therefore, despite setting the interval t between the edge division points P as explained above, the exposure mask pattern 3 becomes complicated, formation of the exposure mask pattern 3 becomes difficult, the mask cost increases, and fabrication of a mask guaranteed against defects becomes difficult.
Further, when using an exposure mask pattern 3 formed in this way to fabricate an exposure mask, the data of the exposure mask pattern 3 (so-called “mask data”) is divided into individual rectangular portions. Therefore, the exposure mask pattern 3 is divided by division lines 7 provided perpendicularly from the edge division points P to the edges, if necessary, and electron beam lithography is performed using each divided pattern 9 portion as a shot area.
However, when forming an exposure mask pattern 3 having “deviation” at the edge portions as mentioned above with rectangular divided patterns 9, divided patterns 9 that have sides smaller than the interval t between the edge division points P are formed. Therefore, despite the interval t of the edge division points P being set as explained above, the number of the divided patterns 9 increases. This becomes a factor inviting an increase of the number of the shots in exposure when producing a mask and deterioration of the TAT (turn-around time).